Digital halftone printing refers to the process of converting a continuous tone image to binary dots for its reproduction by a printing device. The continuous tone image is usually converted by a raster image processor. In some halftoning techniques each pixel of the original image is converted from the continuous tone into a binary dot according to a threshold array of a halftone screen.
The term halftone screen refers to a pattern of printed dots of varying size and shape which is applied to the digital input of an image for printing. Different halftone screens differ by the number of printer pixels which are grouped to form the dots. The size of the dots also varies according to tone. The halftone screens differ by the shape of the patterns and the shape of the dots. The shape of the dots vary and are elliptical, round or square. Also the dot growth is different for the halftone screens. For some halftone screens the size of dots is increased from the dot center. For other halftone screens the dot size is increased in intervals. (see, for example, D. L. Lau and G. R. Arce, “Modern Digital Halftoning”, Marcel Dekker, Inc., 2001, pages 1 to 26).
Prior to printing, a printing device is usually calibrated and a tone-reproduction curve is determined, which defines the relationship between the digital input and the output actually reproduced by the printing device. Generally, individual calibrations are required for each halftone screen.
U.S. Pat. No. 6,484,631 B2 describes a method and system that updates individualized calibrated tone reproduction curves for a printing device. A plurality of predetermined stored calibrated tone-reproduction curves are used, each predetermined stored calibrated tone-reproduction curve corresponding to a distinct combination of substrate and halftone screen. The predetermined stored tone-reproduction curves are related to a reference substrate and reference halftone screen combination. For calibration of the printing device, the reference substrate and reference halftone screen combination is used and a new associated tone-reproduction curve is determined. The predetermined stored tone-reproduction curves are updated by using the new tone-reproduction curve and the relations between the predetermined stored tone-reproduction curves and the reference substrate and reference halftone screen combination.